This invention relates to gas turbine engines and, more particularly, to a low-cost device for increasing the power output of gas turbine engines at low ambient inlet temperatures.
Gas turbine engine power output is related to the temperature of the ambient air entering the compressor portion of the engine. Typically, at rated turbine inlet temperature, as the ambient temperature decreases, power output increases. However, at an established lower limit of temperature (which varies with each engine design) power output may cease to increase and, in fact, may decrease. It is extremely desirable in modern gas turbine engines, particularly those of the turboshaft variety which are used for industrial or marine applications in cold climates, to forestall the onset of power decay with decreasing temperature and, in fact, to reverse the trend if possible.
The problem finds its origin in the inherent characteristics of the components of a gas turbine engine, particularly the compressor and turbine. The compressor is limited by the amount of corrected inlet flow that it can pass, corrected inlet flow being equal to the physical mass flow times a factor proportional to the square root of the ambient inlet temperature (and other factors irrelevant for purposes of this discussion). In a compressor, the corrected flow limit is normally controlled by limiting the compressor's corrected rotational speed. Furthermore, the significant rotational speed characteristic of a gas turbine engine compressor is its corrected rotational speed which equals its physical rotational speed divided by a factor proportional to the square root of ambient inlet temperatures. The turbine, on the other hand, is limited by an actual physical temperature value dependent upon the properties of the materials of which it is fabricated and cooling systems employed, if any.
During operation, as ambient inlet temperatures decrease and corrected speed is increased (as by a throttle advance) to maintain maximum rated turbine inlet temperature, compressor corrected flow and physical mass flow increase rapidly until the flow limit is reached. Power, which is a function of turbine inlet temperature and mass flow, thus increases. However, operation above this flow limit may stall the compressor. Therefore, at even lower inlet temperatures, engine physical speed must be reduced to maintain the maximum compressor flow and stall margine which, in turn, reduces the turbine inlet temperature to less than its rated value and decreases its power output. The problem is one of how rated turbine inlet temperature at maximum compressor flow can be maintained to increase power output at low ambient inlet temperatures. In other words, it is a problem of how the flow-temperature relationship of the gas turbine engine can be varied to modulate power output, particularly at low ambient inlet conditions.
Solutions to this problem have been proposed in the past. For example, it has been proposed to recycle at least a portion of the hot turbine exhaust gases back into the compressor inlet, thereby raising the inlet temperature to increase power at cold ambient temperatures. However, this method cannot show a power gain at low temperatures because the physical mass flow through the engine is reduced at high air inlet temperatures.
In the case of a gas turboshaft engine having a core engine and an independent power turbine downstream of the core engine and driven by the hot gases of combustion, a variable area power turbine nozzle diaphragm of a well-known variety could be used to effect a change in the flow-temperature characteristics of the core engine. Such modulation is apparent to those familiar with such gas turbine engines and the mechanism need not be pursued in detail herein. However, it will also be appreciated and recognized by those familiar with such art that the power turbine nozzle area reduction which would be required for cold-day operation results in reduced compressor stall margin, potentially large shifts in the core engine rotor thrust loads and possible reduction in power turbine efficiency. Additionally, a variable area power turbine nozzle would increase the expense fo designing, controlling and maintaining the engine.
Two other approaches toward obtaining greater cold-day power would be to add an additional stage to the front of the core engine compressor (commonly referred to as "zero staging" the compressor) and to add a variable area capability to the core engine turbine nozzle. However, it should be appreciated that lightweight gas turbine engines for industrial and marine applications are usually derivatives of proven aircraft gas turbine engines and that it is desirable from both the economic and reliability standpoints to maintain commonality of hardware. Both of the aforementioned approaches could result in a virtual redesign of the core engine and are, therefore, undesirable.
It is clear, therefore, that none of the above solutions is entirely satisfactory in solving the low temperature power output problem. What is needed is a low-cost, reliable and efficient method of increasing the power output at low ambient inlet temperatures.